Nitric oxide level modification on antioxidant enzyme activity, antioxidant capacity and pericarp browning of postharvest longan fruit

Abstract

Overproduction of nitric oxide free radical (gNO) potentially causes the reduction in antioxidant defense system, damaging cell components and leading to physiological disorders in postharvest horticultural crops. In this study, the effects of sodium nitroprusside (SNP; NO donor) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO; NO scavenger) on antioxidant enzyme activities, antioxidant capacity and pericarp browning development of harvested longan fruit were investigated. Longan fruits were dipped in distilled water (control), SNP (50 and 100 mM) or cPTIO (100 and 500 μM) for 10 min, then stored at 25 ± 1 °C for 7 d. It was shown that antioxidant enzyme activities including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GPX) and total antioxidant capacity including ABTS and DPPH decreased during storage. These reductions coincided with an increase in pericarp browning (browning index). Enzyme activity and antioxidant capacity assays confirmed our results that NO involved in longan pericarp browning. The activity of SOD, CAT, APX and GPX were reduced by 2.5-71.5%, 30.6-66.1%, 26.7-75.5% and 34.3-68.7%, respectively, after SNP treatment for 7 d but were increased by 2.4-109.6%, 3.8-153.8%, 6.3-47.1% and 31.9-156.5%, respectively, after treatment with cPTIO. In addition, the total antioxidant capacity ABTS and DPPH were reduced by 13.3-66.8% and 17.6-49.4%, respectively, after SNP treatment for 7 d but were increased by 3.6-58.8% and 2.0-128.7%, respectively, after treatment with cPTIO. Treatment with 100 mM of SNP was more efficient in inhibited the activity and capacity of antioxidant than that of 50 mM SNP. In contrast, the cPTIO-treated fruit exhibited opposite results by maintaining antioxidant enzyme activities and antioxidant capacity, corresponding to the reduction in pericarp browning. Treatment with 500 μM of cPTIO was more efficient in increased the activity and capacity of antioxidant than that of 100 μM SNP. The antioxidant potential and development of pericarp browning depended on the applied concentration of SNP or cPTIO. These results revealed that exposure to excessive NO resulted in the reduction in antioxidant potential and the increment of pericarp browning of harvested longan fruit.